1. Field of the Invention
This invention relates to mass flow meters, and more particularly to chip-type temperature sensors and four-sensor bridge circuits for mass flow meters.
2. Description of the Related Art
Numerous different methods are employed to measure the flow rate of gases and liquids. They can generally be divided into two categories: those that measure volumetric flow, and those that measure mass flow.
An example of a volumetric flow meter is a tapered tube through which the gas or liquid travels, displacing a float in the tube. When there is no flow, the float rests at the bottom of the tube, sealing its narrower end. As the fluid flows through the tube, the float rises proportionally to the volumetric fluid flow.
A principal problem with volumetric flow meters relates to the measurement of gas flow rate. Changes in the pressure or temperature of the gas can cause inaccuracies in the flow measurements.
Mass flow meters (MFMs) are conventionally used to operate a valve which controls the flow rate of a fluid through a conduit; the combined MFM and valve is referred to as a mass flow controller (MFC). These devices are used in many systems requiring precise control of gas or liquid flow rate, such as in the semiconductor processing industry to deliver gases whose atoms are used to grow or dope semiconductor materials, where gas flow rates are crucial yield parameters. MFCs for the semiconductor industry are discussed in general in “Results from the workshop on Mass flow measurement and control for the semiconductor industry”, National Institute of Standards and Technology (NIST), on May 15-16, 2000, results published Jul. 20, 2000. An advantage of MFCs over volumetric flow measurement is that mass flow is less susceptible to accuracy errors due to variations in line pressure and temperature. Known types of MFCs include immersible thermal MFCs, thermal MFCs, and differential pressure MFCs.
Thermal MFCs are the most commonly used type of MFC in the semiconductor processing industry. They can be made from relatively inexpensive components, and provide a good compromise between price and performance. With immersible MFCs, one or more sensors are located directly in the flow stream, while with capillary tube MFCs a capillary tube parallels the main fluid conduit, and one or more sensors are provided on the outside of the tube.
In immersible thermal MFCs, an immersed temperature sensor also acts as a heater, heating up as electric current passes through it. The temperature sensor remains at some known constant temperature when the fluid is not flowing. A flowing fluid reduces the sensed temperature, due to the fluid's carrying heat away from it. The magnitude of the sensed temperature drop is proportional to the fluid's mass flow rate. The sensor may be encapsulated for applications where there is a concern about the sensor material contaminating the flowing fluid stream, or itself being contaminated by the fluid.
In an alternate submersible thermal MFC, a heater is immersed upstream and a temperature sensor downstream. The amount by which the fluid temperature at the sensor location rises due to operation of the upstream heater can be correlated with the fluid's mass flow rate.
In capillary tube thermal MFCs, a known fraction of the incoming flow stream is directed through a heated capillary tube, while the remainder of the flow stream by-passes the capillary tube. The tube is heated by metal wire that is wound around its outer surface at an upstream location, with a temperature sensing winding at a downstream location. Platinum wire is typically used because its resistance change, as a function of temperature, is well known, allowing it to act as both a heater and a temperature sensor. Some MFC manufacturers use thin film platinum resistance temperature devices, consisting of a thin layer of platinum on a thin film insulator (typically alumina) that is deposited onto the outer surface of the capillary tube. The platinum thin film layer changes resistance as a function of temperature.
The gas diverted through the capillary tube absorbs some of the heat from the upstream windings. If no gas is flowing, the tube will be heated uniformly and the up and downstream sensors will sense equal temperatures. Once the gas begins to flow through the tube, its heat absorption capacity cools the upstream portion of the tube while heating the downstream portion; the temperature differential increases with increasing gas flow. On-board or remotely located electronics provide an excitation voltage or current for the sensors, and also monitor the sensor response. For example, if a current is applied, the voltage across the winding is monitored so that the winding's resistance is known. Since the resistance of the sensor varies as a known function of temperature, the temperature at the sensor can be determined from its current and voltage.
Thermal MFCs can be either constant current or constant temperature devices. In a constant current device, the temperature sensors are electrically connected as two of the resistive elements in a bridge circuit; the other elements are passive resistors. The constant excitation current is converted to heat by the sensor resistances, providing a uniform temperature gradient along the capillary tube.
In a constant temperature device, the sensors are again connected in a two-sensor bridge circuit, but the MFC electronics provide a constant voltage rather than a constant current to the bridge circuit. A fluid flowing through the tube causes a reduction in the temperature of the upstream sensor, which reduces its resistance (for a positive temperature coefficient sensor), causing more current to flow through it. The increase in excitation current causes the sensor to give off more heat, which replaces the heat lost to the fluid. The additional current is proportional to the fluid's mass flow rate. Platinum is typically used as the sensing element.
While they are in widespread use, presently available MFCs suffer from one or more of the following characteristics: relatively high temperature sensor drift, low sensitivity, long response times, waste associated with the difficulty of handling ultra-fine platinum wire during manufacture, additional electronics required to quantify the electrical responses of low sensitivity temperature sensors, and errors resulting from the circuitry for low sensitivity temperature sensors, when used in conjunction with high sensitivity sensors.